Make js.Bridge aware of string.String for input parameters

Avoids having to allocate small strings when going from v8 -> Zig. Also
added a discriminatory type, string.Global which uses the arena, rather than
the call_arena, if an allocation _is_ necessary. (This is similar to a feature
we had before, but was lost in zigdom). Strings from v8 that need to be
persisted, can be allocated directly v8 -> arena, rather than v8 -> call_arena
-> arena.

I think there are a lot of places where we should use string.String - where
strings are expected to be short (e.g. attribute names). But started with just
document.querySelector and querySelectorAll.

src/cdp/testing.zig

@@ -33,6 +33,7 @@ pub const expectEqual = base.expectEqual;
 pub const expectError = base.expectError;
 pub const expectEqualSlices = base.expectEqualSlices;
 pub const pageTest = base.pageTest;
+pub const newString = base.newString;
 
 const Client = struct {
     allocator: Allocator,